1. Field of the Invention
The present invention generally relates to a delta-sigma modulator, and more particularly to a continuous-time (CT) radio-frequency (RF) bandpass delta-sigma modulator.
2. Description of Related Art
A delta-sigma (ΔΣ) modulator or sigma-delta (ΣΔ) modulator is a feedback system that employs simple circuit blocks to achieve high-resolution output signals. The delta-sigma modulator is widely adapted to electronic circuits such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs) or frequency synthesizers, and is highly attractive in wireless communications due to its simplicity and low-power consumption. The radio-frequency (RF) bandpass delta-sigma modulator is one type of delta-sigma modulator that is well adaptable, for example, to RF signal reception and subsequent digitization without frequency down-conversion.
FIG. 1A shows a block diagram of a conventional continuous-time RF bandpass delta-sigma modulator. FIG. 1B shows an equivalent mathematical model of the block diagram of FIG. 1A. With respect to the circuit implementation of the bandpass filter 10, an LC bandpass filter provides better linearity and low-power consumption as compared to an active-RC resonator mainly for the reason that an operational amplifier of the active-RC resonator is unrealizable under high frequency. However, the delta-sigma modulator using the LC bandpass filter causes a second-order function itself with only one input node. This therefore causes a problem of insufficient control variables.
In order to resolve the insufficient-control-variable problem, a multi-feedback DAC is adopted such that at the same input and output node two different DACs 20 and 22 are used, as shown in FIG. 2, instead of using the single DAC 12 as shown in FIG. 1A. Specifically, the two DACs of different DAC type are utilized to generate different path equations in order to overcome the insufficient control variables at each feedback point. FIG. 3 shows three different types of DACs, that is, a non-return-to-zero (NRZ) DAC, return-to-zero (RZ) DAC and half-return-to-zero (HRZ) DAC, which are commonly used in bandpass delta-sigma modulators. The DAC feedback impulse response dac(t) and the corresponding Laplace transform DAC(s) are both shown in the figure. It can be observed in the figure that the NRZ DAC pulse has only one transition in one sampling period, while the RZ DAC and the HRZ DAC pulses have two transitions in one sampling period.
Nevertheless, adoption of the multi-feedback DAC can cause a relatively serious problem of clock-jitter (e.g., clock-jitter effect). Clock-jitter affects the falling/rising times of the feedback DACs 20 and 22 (FIG. 2), and will degrade the performance of the modulator.
For the reason that the conventional bandpass delta-sigma modulator cannot effectively solve both problems of insufficient control variables and the clock-jitter effect, a need has arisen to propose a novel bandpass delta-sigma modulator in order to overcome the aforementioned problems.